JP2511813B2 - Optical fiber gyro with a light source having a broad spectrum - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a device for stabilizing the average wavelength of a light source having a wide wavelength range of radiated light, particularly a light source used for a rate gyro. The present invention also relates to an optical fiber gyro to which the device is applied.

Conventional technology When the output of an optical system depends on the wavelength,
It is known that the scale factor becomes unstable as the wavelength changes. The largest cause of disturbances is generally temperature changes.

Rate gyros and similar devices are among the fields of application of the present invention.

 The gyro is generally composed of a loop-shaped interferometer.

In a loop or Sagnac interferometer, two beams propagate in the same optical path in opposite directions and interfere at the exit of this optical path. As long as the disturbance in the optical path has the same property in both directions of propagation and the property does not change during the propagation of the light, the two beams undergo the same disturbance and the relative phase does not change. Such a disturbance is called "reciprocal". Since the propagation time in an interferometer is generally very short, the change in disturbance during this time is negligibly small. However, it is not the case when the disturbance is intentionally given.

On the other hand, there are also “non-reciprocal” disturbances. This disturbance has different amplitudes in two directions of propagation. When the direction of this disturbance is completely determined, the physical effect of disturbing the symmetry of space and medium appears.

 Two effects have this property.

One is the Faraday effect, that is, the magneto-optical effect of transmitted light. Due to this effect, when a magnetic field is applied to the optical material, the spin of the electrons inside tends to point in a specific direction.

The other is the Sagnac effect, or relativistic inertial effect. Rotation of the interferometer with respect to the inertial coordinate system breaks the symmetry of the propagation time.

The optical fiber gyro can be realized by rotating the inertial space, and the current sensor based on the current measurement or the magnetic measurement can be realized by applying the magnetic field.

In recent interferometers, the loop is a coil formed by winding a single mode optical fiber. In this way, the radius is reduced. However, the optical path length is proportional to the number of turns of the coil. Therefore, even if the occupied volume becomes very small, the optical path length can be made very long. Its length is usually on the order of 1 km.

Such interferometers are used to measure rotational speed. This rotation speed is obtained by measuring the phase difference between lights propagating in opposite directions in the loop.

As described in detail below, the phase difference between these two lights is proportional to its wavelength. To be more precise, it is proportional to the reciprocal of the wavelength.

The same light beam from the light source is split and propagates in the loop. A semiconductor light source is generally used as the light source.
For example, a high performance light emitting diode can be given. This light source emits light having a wavelength spread centered on the average wavelength. The average wavelength, in turn, depends on the temperature of the light source.

In the past, various methods have been used to stabilize the emission of light. What is common to these methods is that a feedback circuit is provided, but this not only complicates but also requires various adjustments.

The present invention aims to solve such problems.

Means for Solving the Problems According to the present invention, a gyro configured by a loop-shaped interferometer, the optical fiber forming the loop,
A light source that emits light having a wide spectrum centered around a predetermined average wavelength, and separates the light emitted from the light source into light of the same energy and simultaneously inputs them toward both ends of the optical fiber. It is provided with a separating / mixing means for recombining light emitted from both ends, a branching means for branching the recombined light from the optical path of the light emitted from the light source, and a means for detecting the branched recombined light. In the gyro,
A bandpass optical filter centered on the same wavelength as the average wavelength of the spectrum of the light of the light source is provided, and the bandpass optical filter is between the branching means and the light detecting means, in the optical path of the recombined light. A gyro that is installed is provided.

Examples The following description with reference to the accompanying drawings will provide a better understanding of the present invention, as well as features and advantages of the present invention.

Although the present invention will be described with respect to a preferred embodiment, that is, an application to an optical fiber gyro, the present invention is not limited to this.

First, let's review the main features of the optical fiber gyro.

FIG. 1 is a schematic view of an example of a conventional loop interferometer.

In FIG. 1, the loop portion is composed of the single-mode optical fiber 5, but the other portions are provided with conventional optical equipment. The provision of the mode filter 3 makes this interferometer completely reciprocal. The incident beam 11 from the light source 10 is a mode filter 3
After passing through, it is branched into two by the half mirror 4. One beam 12 is incident on the optical fiber 5 via the lens 41. This lens 41 is for focusing the beam 12 on the entrance 50 of the optical fiber 5. The other beam 13 is also incident on the same optical fiber 5 via the lens 42. The lens 42 is for focusing the beam 13 on the entrance 51 of the optical fiber 5. The two beams propagate in the optical fiber in opposite directions and again propagate through the half mirror 4
Come back to. After the two beams pass through the mode filter 3, both beams are separated from the incident beam by the half mirror 2. A beam is partially supplied from the half mirror 2 to the output unit 6. The interference signal is detected by the photodetector 60 installed in the output unit 6.

By using the single mode optical fiber 5 as the optical path, the optical path length is extremely long. However, the optical fiber is generally wound and used as a coil. Therefore, a super-sensitive gyro can be realized.

However, using a beam splitter causes a loss. Therefore, it has been proposed to replace such a discrete optical element with an optical integrated circuit to improve energy loss. However, such a viewpoint is not related to the present invention.

The photodetector 60 is connected to a circuit that processes an electric signal that is the output of the photodetector. However, this circuit is not shown in the figure. The electric signal is obtained by photoelectrically converting the composite beam detected by the photodetector 60 after propagating through the output section 6.

The two beams 12 and 13 interfere with each other depending on their respective phase states. As mentioned above, the phase difference between the two beams depends on the non-reciprocal disturbance exerted on the light propagating in the loop.

 The phase difference ΔΦ is given by the following equation.

In this formula, L is the length of the loop, that is, the length of the optical fiber 5, R is the radius of the loop, that is, the average radius when the optical fiber is wound into a coil, and λ is propagated through the optical fiber 5. Is the wavelength of light in vacuum, C is the speed of light in vacuum, and Ω is the measured value of the rotation speed.

As can be seen from the above equation, the measured phase difference is proportional to the reciprocal of the wavelength λ. If this wavelength λ changes even a little, the scale factor has an effect.

Therefore, the wavelength is directly related to the stability of the emitted light of the light source 10.

By the way, a semiconductor light source is generally used as a light source, but in that case, like other semiconductor elements, it is easily affected by temperature changes. Light having an average wavelength λ ₀ is emitted from the semiconductor light source.

The present invention aims to stabilize this average wavelength without the use of active feedback elements.

In the following, it is assumed that the spectrum of synchrotron radiation has a Gaussian distribution. The present invention is of course not limited to this. In the Gaussian distribution, the distribution of light energy is expressed by the following equation as a function of wavelength.

In this equation, λ is any wavelength in the spectrum, λ ₀ is the average wavelength, and Δλ _S is the half-width of the spectrum.

FIG. 2 is a diagram representing such a spectrum SP _S.

The vertical axis is the energy W of light expressed in an arbitrary scale,
The horizontal axis is the wavelength λ.

Spectrum SP half width [Delta] [lambda] _S of _S, the energy when the 1 the maximum value of the energy is defined as the difference between the wavelengths lambda _1S and lambda _2S of equal across the 1 / e.

As schematically shown in FIG. 3, in the present invention, the bandpass optical filter F _i is connected to the light source S which emits light having the above spectrum. The center wavelength of the spectrum of this filter is still λ ₀ , but the bandwidth is narrower than that of the spectrum SP _S. It is preferable to use an interference filter for this filter.

The interference filter is a filter that uses an interference phenomenon to pass or reflect a spectrum in a certain band. Such a filter is composed of a large number of thin films, and the optical thickness of each thin film is 1/4 of the center wavelength or an integral multiple thereof. The types of materials that make up the thin film, the number of thin films, the thickness
By changing the arrangement of the thin films, the bandwidth can be greatly changed, and the transmittance and reflectance can be increased.

Such filters are commercially available,
There are various configurations. One example is an interference filter called MDI.

This is an interference filter (metal-metal-dielectric).
It is a dielectric interference filter). Such filters are
It has a structure in which semi-permeable metal layers and non-absorption dielectric layers are alternately laminated. The thickness of the dielectric layer plays an important role in determining the wavelength λ of the pass bandwidth having the longest wavelength.

In most cases, filters of the above type are used as bandpass filters.

The wavelength drift in the spectrum of this filter due to temperature changes is mainly due to its geometrical shape change. The drift coefficient is generally 10 ⁻⁶ as described below.
Although it is on the order of / ° C, it is negligibly small compared to the drift of the light source.

The beam F _e emitted by the light source S thus has the same characteristics as the spectrum SP _S shown in FIG.

The transmission energy spectrum of the filter F _i is also shown in FIG.

The half width Δλ _Fi of the spectrum is the difference between the two wavelengths λ _1Fi and λ _2Fi defined in the same manner as in the case of the spectrum SP _S. Therefore, the equation expressing the transmission coefficient of the filter F _i is as follows.

Here, Δλ _Fi = εΔλ _S ... (4) (where ε <1).

If the temperature changes, the output of the filter _Fi is the spectral SP
_{If S} is shifted by wavelength δλ ₀ , beam F _Fi (see Fig. 3)
Have an average wavelength shift of δλ ₀ ′. In this case, λ ₀ ′ and λ ₀
The following relations are established among them.

When ε is a value much smaller than 1, Δλ ₀ ′ ε ² Δλ ₀ .

It can be seen that the change in the average wavelength of the system consisting of "source S and filter F" is much smaller than the change in the wavelength of the beam emitted from the source S, and the object of the present invention is achieved.

Of course, the power of the light detected by the photodetector 60 (FIG. 1) also decreases by a factor ε, but the negative side resulting from this is not as significant as one would expect from the value of ε.

In fact, the S / N ratio is determined by photon noise. The value of this ratio is proportional to the square root of the detected power. Therefore, S / N
The ratio is certainly an important parameter to consider, but the coefficient It only decreases.

 Here are some examples for clarity.

Ga-Al-As (potassium-aluminum-
Arsenic type high performance light emitting diode is used. This light source has a central wavelength of λ ₀ = 830 nm and a half width of Δλ _S = 10 nm.
Emits light with a spectrum of.

If the temperature of the light source is roughly controlled, that is, the change of the order of 1 degree is recognized and stabilized, the ratio ελ ₀ / λ ₀ becomes 3 ×.
^Less than 10 ^-4 .

Therefore, the wavelength drift inherent to the filter (10 ^-6 / ° C)
The above example ignoring is meaningful enough.

The half width at the front of the photodetector is Δλ _Fi = 2 nm, that is, ε
= 0.2 filter is installed, the fluctuation (δλ ₀ / λ ₀ ) of the “source-filter” system becomes smaller than 3 × 10 ⁻⁶ because the coefficient ε ² = 0.04 decreases.

On the other hand, the S / N ratio is a coefficient Only becomes smaller.

According to the present invention, a simple method for successfully compromising the two incompatible factors of stabilizing the scale factor and sensitivity of the system can be obtained.

According to the present invention, the above stabilizing device can be applied to an optical fiber gyro having an interferometer of the type shown in FIG. 1 as a main component. This stabilizer is installed in the passage of the beams 12 and 13, and the position is such that the beams 12 and 13 are in the optical fiber
The position after the exit and recombination but before the photodetector 60. Although not shown in the figure, this stabilizing device is preferably installed at the output 6 of the interferometer of FIG.

The invention is not limited to the particular embodiments described above. The present invention is applicable to any light source with a broad spectrum.

In the above description, a transmission type bandpass optical filter is assumed, but of course a reflection type filter may be used. However, the central wavelength of the spectrum of the reflected light is λ ₀ ,
It is necessary to satisfy the condition imposed on the device of the present invention that its spectral width is narrower than that of the light source.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a conventional interferometer, FIG. 2 is a diagram showing a spectrum of emitted light of a light source and a transmission spectrum of an optical filter, and FIG. 3 is a schematic diagram of an apparatus of the present invention. is there. (Main reference numbers) 2,4 ... Half mirror, 3 ... Mode filter, 5 ... Optical fiber, 6 ... Output section, 10 ... Light source, 11 ... Incident beam, 12,13 ... Beam, 41,42 …… Lens, 60 …… Photodetector, F _i …… Optical filter, S …… Light source, F _S , F _Fi …… Beam,

Claims (4)

(57) [Claims] 1. A gyro composed of a loop-shaped interferometer, comprising: an optical fiber forming the loop; a light source for emitting light having a broad spectrum centered on a predetermined average wavelength; Separation / mixing means for separating emitted light into lights of the same energy and inputting the light toward both ends of the optical fiber at the same time, and recombining light emitted from both ends of the optical fiber; and recombined light from a light source. A gyro equipped with a branching means for branching from the optical path of the emitted light and a means for detecting the branched recombined light, in a band-pass optics centered on the same wavelength as the average wavelength of the spectrum of the light from the light source. A gyro including a filter, wherein a bandpass optical filter is installed in the optical path of the recombined light between the branching means and the light detecting means. 2. The gyro according to claim 1, wherein the bandpass optical filter is an interference filter. 3. The light source has an average wavelength of 830 nm and a half width of 1
The gyro according to claim 1, which is a high-performance 0 nm gallium-aluminum-arsenic type high performance light emitting diode. 4. The gyro according to claim 3, wherein the bandpass optical filter has a spectral width of 4 nm.